1. Field of the Invention
The present invention relates to a sheet conveying apparatus which conveys sheets and an image forming apparatus which has a sheet conveying apparatus.
2. Description of the Related Art
The type of sheet used in recent image forming apparatuses has a wide variety. Further, environment in which an image forming apparatus varies between users, and, with the recent eco-friendly trend, the back faces of sheets which have been used once are more commonly reutilized. Therefore, disturbance factors for conveyance of sheets are significantly increasing. For this reason, it is difficult to completely solve the problem of jamming while sheets are conveyed, and suppressing a jamming incidence as well as how to avoid damages when jamming occurs are becoming more important.
That is, when jamming occurs, it is necessary to safely stop the operation of an image forming apparatus in an easily recoverable state. Typical jamming which may influence a recovery operation from jamming and product performance includes jamming (hereinafter referred to as “accordion jamming”) which is caused by forming a multiple-folded accordion shape of a sheet on a conveying path. The factor which causes this accordion jamming includes cases where a sheet front end cannot proceeds smoothly to a conveying roller in a downstream side due to some factors or a sheet front end sticks to, for example, a difference in level of a conveying path. When accordion jamming occurs, there is a problem that a jammed sheet applies a significant stress to adjacent parts such as guides which form the conveying path. Further, the conveying roller which is conveying the jammed sheet continues pushing the sheet until jamming is detected and driving is stopped, and therefore there is a problem that the sheet is caught by the conveying roller. Particularly when the conveying roller is a fixing roller, once the sheet is caught, a toner image on the sheet is melted by heat, and the sheet faces are adhered and placed in a state where it is very difficult to process the jammed sheet.
A technique is known which detects the growth of an excessive loop which leads to jamming of a sheet by monitoring the loop state of a sheet which is being conveyed based on the position of a sensor lever which is pushed by the sheet to be conveyed and rotates (see Japanese Patent Laid-Open No. 2003-057892). The detecting mechanism of Japanese Patent Laid-Open No. 2003-057892 changes an output at two stages according to the rotation amount of the sensor lever. When a sheet is normally conveyed, the sensor lever which is pushed by this sheet and rotates a small angle, and one output of two sensors becomes on. By contrast with this, when the sheet front end is caught by the conveying guide, conveyance is prevented, and a loop grows excessively or when accordion jamming occurs, the sensor lever is pushed by the sheet and rotates greater than the above small angle, and the outputs of two sensors become on.
Further, a technique is known which detects the sheet thickness (sheet strength) according to the position of the sensor lever which is pushed by a sheet to be conveyed and rotates (see U.S. Pat. No. 5,962,861). When a sheet having a normal thickness is conveyed, the sensor lever is pushed by the sheet and rotates a small angle. By contrast with this, when a sheet which is thicker than the normal thickness is conveyed, the sensor lever is pushed by the sheet and rotates greater than the above small angle, so that the sensor detects that the sensor lever significantly rotates. Further, the sheet thickness is recognized based on the output from the sensor.
However, with the conventional technique for obtaining the above information related to sheets (sheet information), the relative relationship between the movement amount of the sensor lever which is pushed by a sheet and a biasing force which applies a force to the sensor lever in a direction in which the sensor lever abuts on the sheet holds a nearly linear relationship in a movable range of the sensor lever, and therefore there is the following problem.
When, for example, the sheet front end proceeds into the conveying roller on the conveying direction downstream side beyond the sensor lever or when the sheet rear end passes the conveying roller on the upstream side beyond the sensor lever, this shock may be transmitted through the sheet face. Accompanying this phenomenon, the sensor lever is repelled by the sheet face, and the sensor lever is pushed by the sheet and significantly rotates even though the loop of the sheet is not actually excessive or the sensor lever is pushed by the sheet and rotates significantly even though the sheet is not thick, thereby causing false detection of sheet information.
Further, even in case where a sheet is curled, the sensor lever is repelled by the sheet front and rear ends when the sheet front and rear ends pass the sensor lever, and significantly rotates, thereby causing false detection of sheet information.